Slag in Gritty Mix

fortyonenorth(6b)December 3, 2011

Has anyone ever used slag as a substitute for granite in the gritty mix? Here in NW Indiana we have lots of steel mills and, of course, lots of slag. Our town uses a gravelly-textured grade on the roads (instead of sand and/or salt) during the winter. It's probably in the 2-3mm range. I gathered up a bit last year and used it when I was re-potting a rosemary plant. The results were very encouraging. Wikipedia says that slag is used as a phosphorus fertilizer--I'm assuming this would be a finely ground powder, though.

I don't know that I'd use this on food crops but, aside from that, any reasons why this wouldn't be a good choice?

The wholesaler where I buy perlite and sometimes bark and other nursery supplies has a half mile long road covered with slag. I know some growers use it, but the biggest challenge is usually finding it in an appropriate size. It should work fine as a substitute for granite on a size for size basis if it's essentially inert.

My understanding is that slags should be avoided unless you are very familiar with the chemical process used to produce them and their resulting chemical composition. They often contain high levels of heavy metals. This might be why you said you wouldn't use them on food crops - but some heavy metals aren't great for ornamentals as well.

It is my understanding that slags are most often used as a liming ingredient in soils - ie: they are used to raise soil pH. Typical "basic slag" (from steel production) will have a CCE around 50-70 and dolomitic lime would be something like 100.

I don't think you could consider the slag to be "inert" no matter what the size.

I could be wrong - I just looked into it after your question because I was interested but it looks to me like it would mess with the soil pH in unpredictable (and, in the case of the gritty mix, undesirable) ways and might also contain unwanted metals that would be released slowly.

"Steel slags are calcium alumino-silicate oxides. Since they form at the melting point of iron (>2,700 degrees F), most compounds with lower boiling points have been driven off. These compounds contain elements like sulfur, selenium, carbon, cadmium, lead, copper, and mercury. Most of the residuals are encased within a glassy matrix. Fortunately, the matrix is soluble and releases calcium and manganese oxides, which can drive the pH of the dissolving fluid to 10 or 11."

"Fortunately" for some applications - but not so fortunately for you ;) So I believe my initial advice was sound. There are other kinds of slag and even when it comes to steel there are different types based on different stages of the steel refinement.

But really the general principal that Ca compounds are being used to extract out undesirable metals/minerals from the iron ore and that they all end up trapped in a soluble matrix should indicate that slag would preform very poorly in the gritty mix unless you're looking for some absurdly high planting media - but then you'd still be better off with dolomite because you don't have to wonder what kind of metals or minerals are trapped inside...

They just call it "slag" around here. Nothing more specific, I'm afraid. It looks like this (link provided). The use is the same, though I can't say for sure it is exactly what's represented on this particular website.